In social-tagging systems, a user may create a tag (free-form, freely-chosen text) that becomes associated with a resource, such as a URL (Uniform Resource Locator) or a URI (Uniform Resource Indicator). Any user of the system may then use that tag to search for associated resources. Examples of such social-tagging systems or social software systems are IBM's Dogear, and Internet services such as Delicio.us and Flickr.
An experienced user of social tagging systems knows that some tags are already in use—i.e., a specific tag has previously been used by that user, or by other users. Users often create tags for known or anticipated audiences. In some cases, users agree (offline) on a tag to be used for specific types of resources. In other cases, users tend to converge on one or more tags that can become anchor points in an on-going, asynchronous collaboration about those tags or about the topics that those tags refer to. In still other cases, the choice of which tag to use is unstructured (i.e., both plural and singular forms, different verb tenses, etc.) and both experienced and inexperienced taggers struggle with that decision.
How are previously written tags used? There are two major mechanisms. Nearly all social-tagging services provide means for tag-based search of their contents. Systems may keep a log of such searches, optionally including a record of which user searched for which tag. Some social-tagging services also provide the ability to subscribe to a tag. A user may select a particular tag, and request a subscription. Subsequently, the user is notified about any new uses of that tag. The notification methods may include email, RSS (Really Simple Syndication) feeds, or other specific means.